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SPM Course
London, May 2013
Contrasts &Statistical Inference
Ferath Kherif
Normalisation
Statistical Parametric MapImage time-series
Parameter estimates
General Linear ModelRealignment Smoothing
Design matrix
Anatomicalreference
Spatial filter
StatisticalInference
RFT
p <0.05
A mass-univariate approach
Time
Estimation of the parameters
= +𝜀𝛽
𝜀 𝑁 (0 ,𝜎 2 𝐼 )
��=(𝑋𝑇 𝑋 )−1 𝑋𝑇 𝑦
i.i.d. assumptions:
OLS estimates:
��1=3.9831
��2 −7={0.6871 ,1.9598 , 1.3902 , 166.1007 , 76.4770 ,− 64.8189 }
��8=131.0040
=
�� 2= ��𝑇 ��𝑁−𝑝�� 𝑁 (𝛽 ,𝜎2(𝑋𝑇 𝑋 )−1 )
Contrasts A contrast selects a specific effect of interest.
A contrast is a vector of length .
is a linear combination of regression coefficients .
[1 0 0 0 0 0 0 0 0 0 0 0 0 0][0 1 -1 0 0 0 0 0 0 0 0 0 0 0]
𝑐𝑇 �� 𝑁 (𝑐𝑇 𝛽 ,𝜎2𝑐𝑇 (𝑋𝑇 𝑋 )−1𝑐 )
Hypothesis Testing
Null Hypothesis H0
Typically what we want to disprove (no effect).
The Alternative Hypothesis HA expresses outcome of interest.
To test an hypothesis, we construct “test statistics”.
Test Statistic T
The test statistic summarises evidence about H0.
Typically, test statistic is small in magnitude when the hypothesis H0 is true and large when false.
We need to know the distribution of T under the null hypothesis. Null Distribution of T
Hypothesis Testing
p-value:
A p-value summarises evidence against H0.
This is the chance of observing value more extreme than t under the null hypothesis.
Null Distribution of T
Significance level α:
Acceptable false positive rate α.
threshold uα
Threshold uα controls the false positive rate
t
p-value
Null Distribution of T
u
Conclusion about the hypothesis:
We reject the null hypothesis in favour of the alternative hypothesis if t > uα
)|( 0HuTp
𝑝 (𝑇>𝑡∨𝐻0 )
cT = 1 0 0 0 0 0 0 0
T =
contrast ofestimated
parameters
varianceestimate
box-car amplitude > 0 ?=
b1 = cTb> 0 ?
b1 b2 b3 b4 b5 ...
T-test - one dimensional contrasts – SPM{t}
Question:
Null hypothesis: H0: cTb=0 H0: cTb=0
Test statistic:
pNTT
T
T
T
tcXXc
c
c
cT
~ˆ
ˆ
)ˆvar(
ˆ
12
T-contrast in SPM
con_???? image
Tc
ResMS image
pN
T
ˆˆ
ˆ 2
spmT_???? image
SPM{t}
For a given contrast c:
yXXX TT 1)(ˆ beta_???? images
T-test: a simple example
Q: activation during listening ?
Q: activation during listening ?
cT = [ 1 0 0 0 0 0 0 0]
Null hypothesis:Null hypothesis: 01
Passive word listening versus rest
SPMresults:Height threshold T = 3.2057 {p<0.001}voxel-level
p uncorrectedT ( Zº)mm mm mm
13.94 Inf 0.000 -63 -27 15 12.04 Inf 0.000 -48 -33 12 11.82 Inf 0.000 -66 -21 6 13.72 Inf 0.000 57 -21 12 12.29 Inf 0.000 63 -12 -3 9.89 7.83 0.000 57 -39 6 7.39 6.36 0.000 36 -30 -15 6.84 5.99 0.000 51 0 48 6.36 5.65 0.000 -63 -54 -3 6.19 5.53 0.000 -30 -33 -18 5.96 5.36 0.000 36 -27 9 5.84 5.27 0.000 -45 42 9 5.44 4.97 0.000 48 27 24 5.32 4.87 0.000 36 -27 42
1
𝑡= 𝑐𝑇 ��
√var (𝑐𝑇 ��)
T-test: summary
T-test is a signal-to-noise measure (ratio of estimate to standard deviation of estimate).
T-contrasts are simple combinations of the betas; the T-statistic does not depend on the scaling of the regressors or the scaling of the contrast.
H0: 0Tc vs HA: 0Tc
Alternative hypothesis:
Scaling issue
The T-statistic does not depend on the scaling of the regressors.
cXXc
c
c
cT
TT
T
T
T
12ˆ
ˆ
)ˆvar(
ˆ
[1 1 1 1 ]
[1 1 1 ]
Be careful of the interpretation of the contrasts themselves (eg, for a second level analysis):
sum ≠ average
The T-statistic does not depend on the scaling of the contrast.
/ 4
/ 3
Tc
Sub
ject
1S
ubje
ct 5
Contrast depends on scaling.Tc
F-test - the extra-sum-of-squares principle Model comparison:
Null Hypothesis H0: True model is X0 (reduced model)
Full model ?
X1 X0
or Reduced model?
X0 Test statistic: ratio of explained variability and unexplained variability (error)
1 = rank(X) – rank(X0)2 = N – rank(X)
RSS
2ˆ fullRSS0
2ˆreduced
F-test - multidimensional contrasts – SPM{F} Tests multiple linear hypotheses:
0 0 0 1 0 0 0 0 00 0 0 0 1 0 0 0 00 0 0 0 0 1 0 0 00 0 0 0 0 0 1 0 00 0 0 0 0 0 0 1 00 0 0 0 0 0 0 0 1
cT =
H0: b4 = b5 = ... = b9 = 0
X1 (b4-9)X0
Full model? Reduced model?
H0: True model is X0
X0
test H0 : cTb = 0 ?
SPM{F6,322}
F-contrast in SPM
ResMS image
pN
T
ˆˆ
ˆ 2
spmF_???? images
SPM{F}
ess_???? images
( RSS0 - RSS )
yXXX TT 1)(ˆ beta_???? images
F-test example: movement related effects
Design matrix
2 4 6 8
10
20
30
40
50
60
70
80
contrast(s)
Design matrix2 4 6 8
1020304050607080
contrast(s)
F-test: summary
F-tests can be viewed as testing for the additional variance explained by a larger model wrt a simpler (nested) model model comparison.
0000
0100
0010
0001
In testing uni-dimensional contrast with an F-test, for example b1 – b2, the result will be the same as testing b2 – b1. It will be exactly the square of the t-test, testing for both positive and negative effects.
F tests a weighted sum of squares of one or several combinations of the regression coefficients b.
In practice, we don’t have to explicitly separate X into [X1X2] thanks to multidimensional contrasts.
Hypotheses:
0 : Hypothesis Null 3210 H
0 oneleast at : Hypothesis eAlternativ kAH
Variability described by Variability described by
Orthogonal regressors
Variability in YTesting for Testing for
Correlated regressorsV
aria
bilit
y de
scrib
ed b
y V
ariability described by
Shared variance
Variability in Y
Correlated regressors
Var
iabi
lity
desc
ribed
by
Variability described by
Variability in Y
Testing for
Correlated regressors
Var
iabi
lity
desc
ribed
by
Variability described by
Variability in Y
Testing for
Correlated regressors
Var
iabi
lity
desc
ribed
by
Variability described by
Variability in Y
Correlated regressors
Var
iabi
lity
desc
ribed
by
Variability described by
Variability in Y
Testing for
Correlated regressors
Var
iabi
lity
desc
ribed
by
Variability described by
Variability in Y
Testing for
Correlated regressors
Var
iabi
lity
desc
ribed
by
Variability described by
Variability in Y
Testing for and/or
Design orthogonality
For each pair of columns of the design matrix, the orthogonality matrix depicts the magnitude of the cosine of the angle between them, with the range 0 to 1 mapped from white to black.
If both vectors have zero mean then the cosine of the angle between the vectors is the same as the correlation between the two variates.
Correlated regressors: summary We implicitly test for an additional effect only. When testing for the
first regressor, we are effectively removing the part of the signal that can be accounted for by the second regressor: implicit orthogonalisation.
Orthogonalisation = decorrelation. Parameters and test on the non modified regressor change.Rarely solves the problem as it requires assumptions about which regressor to uniquely attribute the common variance. change regressors (i.e. design) instead, e.g. factorial designs. use F-tests to assess overall significance.
Original regressors may not matter: it’s the contrast you are testing which should be as decorrelated as possible from the rest of the design matrix
x1
x2
x1
x2
x1
x2x^
x^
2
1
2x^ = x2 – x1.x2 x1
Bibliography:
Statistical Parametric Mapping: The Analysis of Functional Brain Images. Elsevier, 2007.
Plane Answers to Complex Questions: The Theory of Linear Models. R. Christensen, Springer, 1996.
Statistical parametric maps in functional imaging: a general linear approach. K.J. Friston et al, Human Brain Mapping, 1995.
Ambiguous results in functional neuroimaging data analysis due to covariate correlation. A. Andrade et al., NeuroImage, 1999.
Estimating efficiency a priori: a comparison of blocked and randomized designs. A. Mechelli et al., NeuroImage, 2003.
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